A power transmission apparatus for an engine of a vehicle which includes a hydrostatic continuously variable transmission is known. In the hydrostatic continuously variable transmission, rotation of a crankshaft is transmitted to a transmission through a starting clutch of the centrifugal clutch type, and speed change is performed depending upon a capacity difference between a swash plate hydraulic pump and a swash plate hydraulic motor. Consequently, the rotation of the crankshaft is transmitted at a reduced speed to a driving wheel. In the power transmission apparatus, a speed change driving shaft is rotated by a stepping motor to move a speed change driving member back and forth along the speed change driving shaft to change the angle of the swash plate of the swash plate hydraulic motor to perform speed change. See, for example, Japanese Patent Laid-Open No. 2001-343060.
In the prior art described above, control of the vehicle speed is performed by setting of an engine speed and a ratio (change gear ratio) of a hydrostatic continuously variable transmission. Therefore, upon low speed rotation upon which the torque generated by the engine is low, it is necessary to change the ratio of the continuously variable transmission by a great amount to the low ratio side. Further, in order to prevent a stall of the engine and prevent the vehicle speed from being rendered unstable, it is necessary to change the ratio frequently. As a result, in order to increase the stroke amount of the speed change driving member which moves along the speed change driving shaft, it is necessary to set the length of the speed change driving shaft long. Thus, there is a problem that this increases the scale of the stepping motor and gives rise to an increase in cost. Further, in order to cope with frequent changes of the ratio, it is necessary to raise the durability of the stepping motor and associated cost.
Further, the hydrostatic continuously variable transmission can be used also as a multi-step transmission by fixing the angle of the swash plate of the swash plate hydraulic motor. In this instance, however, similar problems apply also when a driver changes the ratio in accordance with the vehicle speed or the slope of the road surface. Further, significant problems can be associated with an off-road vehicle which runs on an irregular ground which exhibits frequent variations of the road surface situation.